This invention relates to molding and more particularly concerns balanced high-volume venting of a mold cavity.
Various types of elongated thin wall molded plastic articles are presently manufactured and used for many different purposes. Such articles include parts for ballpoint pens and various probe cover sheaths. A widely employed electronic digital thermometer embodies a probe for insertion into a body cavity where temperatures is to be measured. Removable probe sheaths, either disposable or sterilizable, are provided for ready attachment to and detachment from the thermometer probe. Particularly for disposable devices, it is important to maintain a low cost of the sheath. However, the tip of such a sheath must be exceedingly thin in order to provide a rapid heat transfer to the enclosed tip of the probe. Thus a number of prior probe covers have been made in two parts. Typical of such devices are those found in the U.S. pat. Nos. to Mack et al 2,983,385, Ensign et al 3,349,896, 3,500,280 and 3,367,186, Keller 3,469,449, Oudewaal 3,822,593 and VanDeWalker 3,719,396.
In a co-pending application of Russel T. Gilbert et al, Ser. No. 705,136 filed July 14, 1976 for Method and Apparatus for Molding Elongated Thin Wall Articles, there is described an unique method and apparatus for making thermometer probe covers which have a length of four inches, an average maximum diameter of considerably less than one-half inch, and a wall having a thickness measured in thousandths of an inch. The disclosure of such co-pending application is incorporated herein by this reference as though fully set forth. For commonly used injection molding materials, such as polyethylene, for example, heat characteristics that are required to obtain an acceptably short thermometer probe response time, dictate a wall thickness at the temperature sensing tip of the cover of no more than about 0.020 inches. Preferably the wall thickness of the tip is 0.010 inches.
Two major problems have provided obstacles to the molding of such thin wall covers as integral devices. These problems are the (a) maintaining of proper relative position of the long thin cantilevered core within a female mold cavity (the tip of the core must be symmetrically positioned within the cavity to provide an annular cavity space around the core of 0.010 inches) and (b) the attendant problem of adequately and uniformly venting cavity gases that are necessarily displaced during injection of the molding material.
In this type of manufacture, material at a temperature of approximately 480.degree. F. is injected under pressures of many tons, typical injection pressures being significantly greater than 10,000 pounds per square inch. A slight departure from a circumferentially balanced condition of flow and pressure of the entering injected material exerts lateral forces on the long slim cantilevered core. These lateral forces cause the core to bend to an extent that makes the wall of the article unacceptably thin, or to an extent where the tip of the core contacts the side of the cavity and thus leaves a hole in the finished article.
In thermometer probes covers where the article is to be inserted into the human body, it is essential that there be no roughness, uneveness, or other irregularity on the outer surface of the article and especially upon its tip. It is partly for this reason that injection must occur at a point remote from the tip and that the core must be cantilevered, being entirely free at its tip to provide optimum smoothness at this portion of the finished article. Further, the venting, if it occurs at the tip, must be of such a nature as to leave little or no flash and little or no roughness on the tip of the finished article.
The significance of the problem of bending of the cantilevered core may be understood when it is noted that for an article having a wall thickness of 0.010 inches, deflection of the core tip of no more than 0.005 inches, can be tolerated. This is so because a commonly employed plastic cannot be forced to flow through a cavity space of less than 0.005 inches. Accordingly, if the core tip should deflect more than this amount in an arrangement for making articles having nominal wall thickness of 0.010 inches, the article tip cannot be formed because no plastic can reach this area. Nevertheless, a typical steel core, precisely positioned and aligned to be concentric with a cavity may deflect as much as 0.004 inches under forces exerted during conventional injection. This leaves the very difficult tolerance of 0.001 inches for the total of all other factors that contribute to errors in concentricity of core and cavity. Such other factors, in addition to core deflection, include initial centering of the core mounting, actual relative dimensions of the core and cavity diameters, clearance of the shank of the core at the mounting and the angle of the core axis (the degree of perpendicularity of the core). All of these other displacing factors must total not more than 0.001 inches where a common core pin is employed and where it may be expected to bend as much as 0.004 inches.
Another significant problem which contributes to the above-mentioned core deflection problem is adequately and symmetrically disposed venting. As the high pressure heated material is injected rapidly into the core space, defined between the female cavity and the male mold core, air witin the cavity must be rapidly exhausted. Accordingly, vents are provided to accommodate displacement of this air. Nevertheless, although the vents must be of such size as to allow escape of air, they must also prevent or minimize the flow of plastic therethrough. Any flow of the hot plastic through the vent holes causes undesirable flash. Thus venting passages of a size sufficient to handle a high rate of flow of the exiting gases must be provided and yet the venting orifices communicating with the space within the cavity must be sufficiently small.
Balanced venting is an aspect of great importance in the molding of thin wall articles. The venting must be precisely balanced about the cavity and about the core to insure that venting of the gases does not take place more rapidly at one side of the core than the other. If the gas vents more rapidly at one side, that side will fill more rapidly. This may result in one side of the cavity, on one side of the core, containing high pressure liquid injected material before any material has reached the other side of the core. Circumferentially unbalanced inflow of injected material will cause unacceptably large lateral and bending forces on the core and prevent formation of an acceptable molded article.
The method and apparatus of the above-identified co-pending application handles the problem of balanced venting by providing vent apertures at both the end of the cavity in the female mold part and by providing a venting bore through the entire length of the core. To maintain the vent apertures of sufficiently small individual size, vent pins are provided in the vent bores of both the cavity and the core itself to thereby provide an adequate volume of venting. However, these pins require high tolerance precision manufacturing techniques to insure that they are symmetrically positioned, since any asymmetrical positioning of the core pins will result in asymmetry of the venting and undesirable core deflection.
Porous material such as porous plastic and sintered metal, have been used in molds in the past, as shown in the U.S. Pat. Nos. to Kimura et al 3,804,566, Manning et al 3,329,198, Havens 3,254,981, Martin, Jr. 3,078,508, Garvey, Jr. 1,983,976, Carter 965,869 and Tanie 3,822,857. However, these patents do not show or suggest the molding of long thin wall articles and do not teach vent configurations sufficient to provide balanced high-volume venting at the tip of a long narrow cavity.
The patent to Schultz U.S. Pat. No. 2,378,586 shows a longitudinally split two-piece vent employed in the molding of hollow articles in which the two pieces collectively provide an aperture smaller than 0.003 inches for venting through the mold core. This configuration does not itself provide adequate balanced venting for manufacture of articles of the type described herein.
Accordingly, it is an object of the present invention to provide venting for a mold of the type described that is of sufficient volume and precision of balance to avoid or minimize problems of prior art devices.